1. Field of the Invention
The present invention relates to magneto-optic memory devices, and more particularly to magneto-optic memory devices, such as magneto-optic discs and magneto-optic cards, which are adapted to record data thereon and reproduce or erase the data therefrom when exposed, for example, to a laser beam.
2. Description of the Prior Art
Active research has been conducted in recent years for the development of magneto-optic memory device such as optical memory devices adapted for recording, reproducing and erasing data. Especially attention has been directed to devices wherein a rare earth-transition metal alloy thin film is used as the memory medium because the recording bits are free of the influence of the grain boundary and further because the film serving as the memory medium can be formed over a large area relatively easily. However, the magneto-optic memory devices incorporating the rare earth-transition metal alloy thin film as the memory medium generally fail to exhibit satisfactory magneto-optic effects (Kerr effect and Faraday effect) and therefore remain to be improved in the S/N ratio of reproduction signals. To overcome this problem, the so-called reflective film structure is employed for magneto-optic memory devices as already disclosed, for example, in Unexamined Japanese Patent Publication SHO 57-12428 and Applied Optics, Vol. 23, No. 22, p. 3472-3978.
FIG. 3 is a partial view of a vertical section showing a conventional magneto-optic memory device having a reflective film structure. With reference to the drawing, indicated at a is a transparent substrate of glass, polycarbonate resin, epoxy resin or the like, at b a transparent dielectric film having a higher refractive index than the substrate a at c a rare earth-transition metal alloy thin film, at d a transparent dielectric film, and a e a light reflecting film of metal.
With a memory device having the above construction, the rare earth-transition metal alloy film c is so thin that the light incident on the film c partly passes through the film. Consequently, reproduction light exhibits the Kerr effect due to the reflection at the surface of the film c and, in combination therewith, the Faraday effect which is produced by the portion of the light passing through the film c, reflected at the reflecting film e and passing through the film c again. This results in a Kerr rotation angle which is apparently several times as great as the angle afforded by the Kerr effect only. For example, the device shown in FIG. 3 exhibits an increased apparent Kerr rotation angle of 1.6 degrees (although a single GdTbFe film is about 0.3 degree in this angle) when the device comprises a glass substrate as the transparent substrate a, AlN film as the transparent dielectric film b, GdTbFe film as the rare earth-transition metal alloy film c, AlN film as the transparent dielectric film d and Al film as the light reflecting film e.
Nevertheless, the rare earth-transition metal alloy thin film is very susceptible to oxidation in the presence of moisture and loses the inherent magneto-optic characteristics thereof on oxidation. Especially with the device of the basic reflective film structure described above wherein the alloy film c needs to transmit light therethrough, the film is more prone to the influence of oxidation because the film must have a considerably decreased thickness and also because the film is left exposed at its end. In other words, the device has the problem of being poor in long-term reliability.
Accordingly, the device of reflective film structure described above is used usually with another substrate laminated to the reflecting film with an adhesive layer. The adhesive layer is used for covering the entire magneto-optic memory layer comprising the dielectric film b, alloy film c, dielectric film d and light reflecting film e and also for adhering the other substrate.
When the device is in this form, it is expected that the magneto-optic memory layer including the alloy film will be held out of direct contact with the atmosphere, with the alloy film protected against oxidation, so as to assure the device of reliability.
Nevertheless, the structure thus sealing off the device from the atmosphere still encounters difficulty in assuring long-term reliability.
The object of the present invention is to overcome the foregoing problem and to provide a magneto-optic memory device of reflective film structure which exhibits outstanding magneto-optic characteristics over a prolonged period of time.